This invention relates to carriers for use in developer formulations which charge electroscopic powders tirboelectrically. These carriers are useful in electrophotographic processes for developing latent electrostatic images in which a colored toner carried by the carrier particle is caused to be attracted from the carrier particle to develope the latent electrostatic image.
In the electrophotographic process it is necessary to use a carrier for the toner in order to produce an electrostatic charge upon the toner particles. Various kinds of developing processes are known including cascade, powder cloud and magnetic brush processes. In each of these processes it is necessary that the carrier used have certain triboelectric properties so that it is capable of imparting to the toner particles an electrostatic charge of the proper polarity and magnitude. Where uncoated carrier particles are used it has been necessary to select a toner having the desired triboelectric properties. Recently, it has been found that the carrier particles can be coated with various types of polymeric coatings to permit variation in the triboelectric properties thereof. One such method is disclosed in U.S. Pat. No. 3,811,880 to Luther C. Browning, assigned to the same assignee as this invention.
Although polymeric coatings of this type enable a certain degree of control of the triboelectric properties of the developer mix, it has been found that in use in the environment of electrophotographic reproduction machines such carrier particles are subject to aging which limits their effectiveness. Wearing away and removal of part of the polymeric coating upon the surface of the carrier particles is another problem encountered. This may result in undesired abrasion of the photoconductive surface used for imaging and also cause bias shorting.
Another problem inherent in the use of polymeric coatings for carrier particles is the phenomenon known as "bound toner". Through a mechanism which is not clearly understood prolonged usage of developer mixes including polymeric coated carrier particles results in toner being adhered onto the surface of the coated carrier causing a decrease in the effectiveness of the toning process and hence in the overall development of the images being reproduced.
More recent developments include use of perfluoro carboxylic acid coated carriers which impart a positive triboelectric charge to electroscopic powders with which they are mixed as described in U.S. Pat. No. 3,922,381 to Pabitra Datta and the use of molybdenum disulfide and similar materials which are effective to improve the fatigue resistance of developer mixes as described in copending application Ser. No. 570,279, filed Apr. 21, 1975, by Virgil W. Westdale and Charles A. Kumins, both assigned to the assignee of this invention. However, the triboelectric properties, abrasion resistance and longevity of such materials leaves something to be desired.